Debenzolizing wash oil



Jan. 26, 1960 E. J. HELM DEBENZOLIZING WASH OIL Filed Jan. 31, 1957 x mn a BENZOLIZED WASH 01 L INVENTOR. 0 (rm F0 hie-4M a 7- TOEA/E, )4

United States Patent C) 2,922,751 DEBENZOLIZING WASH 011'.-

Edward I. Helm, Pittsburgh, Pa., assignor to Koppers Company, Inc., acorporation of Delaware Application January 31, 1957, Serial No. 637,3953 Claims. Cl. 196-99) This invention relates generally to the removal ofbenzene and its homologues from an absorbent liquid.

The vapors of benzene and its homologues are usually recovered from thegases from coke ovens by a process wherein the gases are contacted withan absorbent liquid to remove these vapors from the gas; thereafter theabsorbent liquid is subjected to distillation to free the henzene andits homologues from the liquid; and the liquid residue from thedistillation reused as absorbent liquid for further contact with thegas. The contacting of the liquid and gas is generally known asscrubbing; the absorbent liquid as wash oil; and the recovered benzeneand its homologues as light oil; the liquid leaving the scrubber andmore or less saturated with benzol at the temperature and partialpressure of the gas entering the scrubber as benzolized wash oil; andthe distillation of the absorbent liquid as stripping; and the residuefrom the stripping tower as debenzolized Wash oil.

The wash oil may be a petroleum distillate and have a boiling range ofaround 300 to 360 C. at atmospheric pressure, and the light oil may havea boiling range of around 78 C. to 200 C.; yet if the benzolized washoil be subjected to ordinary distillation at atmospheric pressure,. itmight begin to boil at 250 C. for a solution of 2% of light oil andwould have to be heated to around 300 C. or more to remove all of thelight oil from the wash oil. In order to remove the light oil at a lowtemperature, the system is converted from one of two components, i.e.,wash oil and light oil, to one of three components by injecting livesteam into the mixture. Thereafter the total pressure is the sum of thepartial pressures of the components; but, because the distillation iscarried out above the normal boiling point of water, there is no waterpresent in the liquid phase and the presence of steam does notsubstantially change the partial pressures of the other components; andeven if liquid phase water were present, the partial pressures ofthe-other components would not be changed materially because of the verylow solubility of water in the oil. Since the vapor pressure of thelight oil is so great as compared with the vapor pressure of the washoil, the vapors leaving the still will be essentially light oil andwater vapors. The quantity of light oil vaporized by a given quantity ofsteam under the foregoing conditions is the product of the ratio oftheir partial pressures at the distillation temperature and the ratio oftheir molecular weights. Thus, the temperature required for thedistillation may be varied as desired by regulating the quantity ofsteam introduced into the distillation. The higher the temperature atwhich distillation takes place, the smaller will be the quantity ofsteam required for the distillation.

The practice heretofore has been to conduct the distillation at therelatively low temperature of between 100 and 140 C. and usually at115-l30 C., thereby utilizing a large quantity of steam to remove thelight oil from the wash-oil, as for example 0.6-0.8 pound ofdirect steamper gallon of wash oil stripped. It was customary,

therefore, to partially condense the vapors from the stripping tower toremove all the excess water vapor and to recover the heat therefrom byindirect heat exchange with the benzolized wash oil, before the vaporswere conducted to a rectifier where the vapors were separated byfractional distillation into the desired hydrocarbon fractions.

I have found that by stripping the light oil vapors from the wash oil atthe elevated temperature of between and 200 C. and advantageously at 190C., the quantity of water vapor required may be maintained low enoughthat the vapors from the stripping tower may be conducted directly tothe rectifying tower without the conventional intermediate partialcondensation of the vapors and yet the rectifier can be operated in anormal manner. Such operation presented a problem,

however, in purifying the wash oil.

As is well known, impurities tend to build up in the wash oil When thewash oil is used in the cyclic process of absorbing the light oil fromcoke oven gas by the wash oil, stripping the light oil from the washoil, and reusing the wash oil for light oil absorption; this tendencybeing enhanced when the stripping operation is carried out at a hightemperature. A common method of purifying the wash oil has involvedcontinuously vaporizing a portion of the wash oil with the steam whichis to be used for the debenzolizing of the Wash oil, flowing theresulting vapors which are comprised mainly of steam saturated with oilvapors to the stripping tower to supply the stripping steam for removingthe light oil from the wash oil,

and discharging from the purifier the residue which is comprised mainlyof relatively non-volatile impurities. Thus, the problem presented isthat when the tower is operated at a high temperature, the quantity ofwater vapor or steam necessary to strip the light oil from the wash oilis not sufficient to vaporize that quantity of wash oil which isrequired to be vaporized in the purifier to maintain the necessarydegree of purity of the wash oil recycled, if the purifier is operatedat the same temperature as normal in the past, which usually representsthe highest temperature obtainable by indirect heat transfer from steamvcondensing at the pressures of 150 to 200 pounds per square inch gaugewhich is usually found in coke plants.

An object of the present invention, therefore, is to provide a novelsystem for removing the light oil from a benzolized wash oil whilemaintaining the required degree of purity of the wash oil being recycledfor further benzolizing.

' Another object of the invention is to provide a novel system forstripping the light oil from a benzolized wash oil and fractionallydistilling the light oil without intermediate condensation of the vaporsfrom the stripper.

A further object of the invention is to debenzolize a benzolized washoil by a novel process and apparatus wherein light oil is stripped fromthe benzolized wash oil at an elevated temperature by a mixture of steamand oil vapors resulting from the distillation of a portion of the washoil from residue containing a large percentage of non-volatileimpurities in a purifier with steam; the

The present invention contemplates a novel arrangement wherein the lightoil is stripped from benzolized wash oil and sent to a rectifier forseparation into desired fractions without intermediate partialcondensatiomofl the vapors, the steam for this stripping of light oilfrom the wash oil being supplied by a mixture of steam and wash oilvapors derived from the distillation of a portion of the wash oil, theheat for the latter distillation being provided by a separateheatsource.

The above and 'further objects and novel features of the invention willappear "more fully from the detailed description when'the same is readin connection with theaccompanying drawing. his to be expresslyunderstood, however, that the drawing is not intendedas a definition ofthe invention but is for purposes or illustration only..

The single sheet of drawing schematically illustrates an embodiment ofthe present invention 'for removing light oil from the wash oil andseparating the light oil into distillation fractions while maintaining apredetermined degree of purity of the wash oil.

The gas from the coke oven is usually contacted with the wash oil inlarge scrubbing towers by flowing the gas upwardly through the tower andthe oil downwardly; Such apparatus is conventional, does notconstitute'a part of the invention, and, therefore, is not shown.

In accordance with the present invention, the benzolized wash oil from asuitable scrubber (not shown) is conducted through a conventionalcountercurrent oil-to-oil heat exchanger where the benzolized wash oilis heated and debenzolized wash oil is cooled. The debenzolized wash oilis further cooled in a conventional water cooler 11 and returned to thescrubber. The benzolized wash oil, on the other hand, is flowed throughasuitable steam or fired heater 12, where the temperature of the oil israised to l60-200 C. and preferably to l70-190 C. The hot benzolized oilis introduced into a conventional bubble-cap tray type stripping tower13 at an upper portion of the tower. As the wash oil descends throughthe tower it is subjected to a countercurrent upward flow of steam whichremoves the light oil as vapors from the wash oil. This mixture of steamand light oil vapors flows directly from tower 13 by way of conduit 15to the approximate middle of rectifier column 17 wherein the light oilis separated into tow fractions; a benzol, toluol, xylol fractionboiling below about 145 C. and a heavy solvent and naphthalene fractionboiling above about 150 C.

Rectifier column 17 may be of a conventional bubblecap tray type withthose trays which constitute the stripping section being fitted withsteam coils arranged for immersion in the liquid. Rectification isbrought about by the provision of a reflux condenser 19 and a fluidseparator 20. The condensate from a condenser 19 is led to separator 29where the water is separated from the benzol fraction. benzol fractionis'divided into two portions, one portion being used as product andanother portion being returned to the rectifier as reflux. A largeportion of the steam originally present in the vapor mixture that is'fed to the rectifier is condensed through direct heat exchange with thelight oil reflux on the trays of the rectifier. Accordingly, the liquidis withdrawn from the tray above the feed, and the water separated fromthe oil in separator 21. The water is discharged to waste and the oil isreturned to the feed plate.

A purifier 31 is provided to remove from the wash oil the polymerizedand non-volatile impurities which tend to accumulate and build up in thewash oil as the wash oil 'is circulated through the scrubbing andstripping towers. This purifier may he of a conventional steamdistilling type unit having at its lower portion a conduit 33 throughwhich steam is introduced, a conduit'35 through which a portion of thewash oil is withdrawn from the stripping -col umn to be vaporized thesteam, a conduit 36 through which the mixture of steam and purified washoil vapors is sent to the stripping tower washoil, and :a conduit 3tthrough which the residue or The water is sent to waste; and the 7quantity of steam is required and the vapors from stripping tower 13 canbe sent directly to the rectifying column 17 without intermediatecooling.

In conventional arrangements as known heretofore, the temperature of theoil purifier is held at to 200 C. by indirect heat exchange with highpressure steam. To vaporize the same quantity of oil with the smallerquantity of steam as used in accordance with the present invention,however, requires purifier 31 to be maintained at atemperature ofbetween 210 and 250 C., and the steam pressures ordinarily available atcoke plants are not high enough to provide the temperatures by indirectheat exchange. In order to maintain these higher temperatures in thepurifier and to supply the latent heat of vaporization of the wash oilat these temperatures, there is provided a heater 50 which may, forexample, be a conventional fired heater. Thus, in accordance with thepresent invention, the steam entering purifier 31 is passed throughheater 50 and superheated to a predetermined extent and/or the residuefrom purifier 31 is recirculated through heater 50 and raised to atemperature greater than the temperature necessary for vaporizing therequired' amount of wash oil when brought into direct contact with theamount of stripping steam to be used. The excess temperature of the washoil provides the latent heat of vaporization of the wash oil to bevaporized. Thus, by way of the heating supplied externally of thepurifier, the required quantity of oil may be purified despite'the smallamount of steam required. The foregoing has presented a novelarrangement for removing the light oil from benzolized wash oil prior tothe recirculation of the wash oil to the scrubber. The stripping toweris operated at a high temperature so as to increase the economy ofoperation without reducing the efficiency or building up impurities inthe wash oil. Provision is made for supplying heat at a high temperaturelevel to the purifier wherein the impurities are removed from the washoil so that the required quantity of wash oil may be purified despitethe small amount of steam required to strip the light oil from the washoil at the high temperatures of the present invention.

Various changes and modifications may be made in the particular detailsof the novel arrangement of this invention without departing from thescope of the appending claims.

What is claimed:

1. Apparatus for debenzoli'zing a benzolized wash oil by removing thelightoil from said benzolized wash oil with steam while purifying thewash oil for reuse, comprising a tower adapted to receive saidbenzolized oil and said steam for removing the light oil as a vapor fromsaid wash oil with said steam, a rectifier adapted to receive said vapordirectly from said tower for fractionally distilling said vapor, apurifier adapted to receive a portion of said wash oil from said tower,means for applying direct steam to said portion so that the wash oil ofsaid portion is vaporized to produce a mixture or water and oil vaporsand any non-volatile matter remains as residue and including a heaterlocated exterior of said purifier for superheating said steam; a conduitfor applying said mixture to said tower to supply said steam forremoving the light oil from said benzolized wash oil, and means forcirculating a quantity of said residue and oil from said purifierthrough said heater so as to receive additional heat whereby a greaterportion of said oil may be vaporized than is possible with the quantityof direct steam applied to said purifier.

2. Apparatus for removing the light oil from ben zolized wash oil Whilepurifying the wash oil for return to a scrubber, comprising a tower forreceiving said benzolized oil and steam for removing the light oil as avapor from said wash oil, rectifier means for fractionally distillingsaid vapor, said rectifier including refiux means, a purifier adapted toreceive steam and a portion of said wash oil from said tower forvaporizing said portion with the steam to produce a mixture of steam andoil vapors while leaving any non-volatile matter as residue, means forapplying said mixture to said tower to supply the steam for removing thelight oil from said wash oil, heating means located exterior of saidpurifier for superheating said steam before entering said purifier forapplying additional heat to said portion whereby a greater portion ofoil is vaporized and means for circulating a quantity of said residueand oil from said purifier through said heater so as to receiveadditional heat whereby a greater portion of said oil may be vaporizedthan is possible with the quantity of direct steam applied to saidpurifier.

3. Apparatus for removing the light oil from benfrom said benzolizedwash oil at temperatures of from to 210 C. by said stripping steam, theamount of stripping steam required decreasing as the temperature of thewash oil in the tower is increased, a purifier for receiving a portionof said wash oil to be purified of impurities, means for applying directsteam to said purifier to vaporize said portion whereby a mixture ofwater and oil vapors is produced while any non-volatile impuritiesremain as residue, said steam applying means including a heater locatedexterior of said purifier for superheating said steam, means forapplying said vapor mixture to said second inlet to supply saidstripping steam to said tower, and means for circulating a quantity ofsaid residue and oil from said purifier through said heater for applyingadditional heat to said quantity of residue and oil at a temperaturelevel of between 210 and 250 C. whereby a constant portion of wash oilmay be purified as the temperature of said wash oil in said tower israised and less stripping steam is required for removing the light oilfrom said benzolized oil.

Chemical Engineers Handbook, third edition, 1950, pp. 582 and 583.

1. APPARATUS FOR DEBENZOLIZING A BENZOLIZED WASH OIL BY REMOVING THELIGHT OIL FROM SAID BENZOLIZED WASH OIL WITH STEAM WHILE PURIFYING THEWASH OIL FOR REUSE, COMPRISING A TOWER ADAPTED TO RECEIVE SAIDBENZOLIZED OIL AND SAID STEAM FOR REMOVING THE LIGHT OIL AS A VAPOR FROMSAID WASH OIL WITH SAID STREAM, A RECTIFIER ADAPTED TO RECEIVE SAIDVAPOR DIRECTLY FROM SAID TOWER FOR FRACTIONALLY DISTILLING SAID VAPOR, APURIFIER ADAPTED TO RECEIVE A PORTION OF SAID WASH OIL FROM SAID TOWER,MEANS FOR APPLYING DIRECT STREAM TO SAID PORTION SO THAT THE WASH OIL OFSAID PORTION IS VAPORIZED TO PRODUCE A MIXTURE OF WATER AND OIL VAPORSAND ANY NON-VOLATILE MATTER REMAINS AS RESIDUE AND INCLUDING A HEATERLOCATED EXTERIOR OF SAID PURIFIER FOR SUPERHEATING SAID STEAM, A CONDUITFOR APPLYING SAID MIXTURE TO SAID TOWER TO SUPPLY SAID STEAM FORREMOVING THE LIGHT OIL FROM SAID BENZOLIZED WASH OIL, AND MEANS FORCIRCULATING A QUANTITY OF SAID RESIDUE AND OIL FROM SAID PURIFIERTHROUGH SAID HEATER SO AS TO RECEIVE ADDITIONAL HEAT WHEREBY A GREATERPORTION OF SAID OIL MAY BE VAPORIZED THAN IS POSSIBLE WITH THE QUANTITYOF DIRECT STEAM APPLIED TO SAID PURIFIER.